Two components of coupling calcium in single ventricular cell of rabbits and rats

A system for rapid superfusion with simultaneous measurement of contractile amplitude of single adult rat and rabbit ventricular cells is used to measure cellular response to alterations of the superfusate achieved in less than 0.3 s. The time course of contractile response of the cells to extracellular Ca concentration [( Ca]o) depletion and repletion identifies "fast" and "slow" cellular pools of Ca that contribute to contraction. The fast pool can be totally depleted or repleted within a single diastolic period. Depletion of this pool completely eliminates contraction in both rat and rabbit cell. Experiments using dimethonium to investigate the effect of Ca in the diffuse double layer and dodecyl sulfate to specifically augment sarcolemmal fixed-negative charge indicate that sarcolemmal binding sites may represent a major fraction of the fast pool. At 1 mM [Ca]o, this pool is functionally saturated in the rat but not nearly saturated in the rabbit. After 10 min Ca depletion more than 60 s of Ca repletion are required to restore full contraction amplitude. This indicates the presence of a slow pool of Ca that contributes to contraction. This pool, at 1 mM [Ca]o, is nearly functionally saturated in the rabbit but not nearly so in the rat. The responses to Ca depletion and repletion, Na depletion and repletion, and 1 microM ryanodine indicate that the contribution of Ca to contraction from the slow pool is much greater in the rat than in the rabbit and that its cellular locus is probably the sarcoplasmic reticulum.